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Pikaia
Pikaia is an extinct animal known from the Middle Cambrian fossil found near Mount Pika in the Burgess Shale of British Columbia. It was discovered by Charles Walcott and was first described by him in 1911. Based on the obvious and regular segmentation of the body, Walcott classified it as a polychaete worm. It resembles a living chordate commonly known as the lancelet and perhaps swam much like an eel. During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens in the chordates, making it perhaps the oldest known ancestor of modern vertebrates, because it seemed to have a very primitive, proto-notochord. Further, the status of Pikaia as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates; further, its tentacles are unknown from other vertebrate lineages. The presence of earlier vertebrates in the Chengjiang, including Haikouicthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument, but the presence of tentacles is still intriguing, and the organism cannot be conclusively assigned even to the vertebrate stem group. in Washington, DC]] Averaging about 1½ inches (5 cm) in length, Pikaia swam above the sea floor using its body and an expanded tail fin. Pikaia may have filtered particles from the water as it swam along. Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate. Only 60 specimens have been found to date. Discovery P. gracilens was discovered by Charles Walcott and first described by him in 1911. It was named after Pika Peak, a mountain in Alberta, Canada. Based on the obvious and regular segmentation of the body, Walcott classified it as a polychaete worm. During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens among the chordates, making it perhaps the oldest known ancestor of modern vertebrates. He did this because it seemed to have a very primitive, proto-notochord, however, the status of Pikaia as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates 2 (although characteristic of other cephalochordates); further, its tentacles are unknown from other vertebrate lineages.2 The presence of earlier chordates among the Chengjiang, including Haikouicthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,3 but the presence of tentacles remains intriguing, and the organism cannot be assigned conclusively, even to the vertebrate stem group. Its anatomy closely resembles the modern creature Branchiostoma. Averaging about 1 1⁄2 inches (3.8 cm) in length, Pikaia swam above the sea floor using its body and an expanded tail fin. Pikaia may have filtered particles from the water as it swam along.needed Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate.needed Only 60 specimens have been found to date. Description Pikaia was a primitive creature that lacked a well defined head, and was typically less than 2 inches (5 centimetres) in length. It swam in the mid-Cambrian seas, and is closely related to the ancestor of all animals with backbones (vertebrates), from fish to amphibians, reptiles, birds, and mammals. Pikaia is one of the most interesting of the multitude of animal fossils found in the famous Burgess Shale in the mountains of British Columbia, Canada. Until recently, there was no comprehensive account of Pikaia's anatomy. However, Conway Morris and Caron (2012, see further reading) have now published an exhaustive description based on all 114 of the known fossils. They found some new and unexpected characteristics and considered most of these to be primitive features of the first chordate animals. On the basis of these findings, they built a new scenario for chordate evolution. Subsequently, Mallatt and Holland (2013, see further reading) summarized the entire history of Pikaia research, considered Conway Morris and Caron's descriptions, and concluded that many of the newly recognized characters are unique specializations (already divergent in the Cambrian) not helpful for establishing Pikaia as a basal chordate. When alive, Pikaia was a compressed, leaf-shaped animal. It swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels. Fish inherited the same swimming movement, but they generally have stiffer backbones. Pikaia had a pair of large head tentacles and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the living lancelet. This primitive marine creature shows the essential prerequisites for vertebrates. The flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail. Unlikely ancestor See also * Metaspriggina Further reading Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton, New York, NY. Conway Morris, Simon. 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, New York, New York. References External links * An artist's rendering * Fossils of the Burgess Shale - Middle Cambrian * http://www.ucm.es/info/zoo/Vertebrados/JADiaz/evolucion.htm (Spanish) Category:Cambrian animals Category:Burgess Shale fossils Category:Transitional fossil Category:Fish Category:Fish stubs